Conventional valves, such as those for precision pneumatic driving units and controlled pressure gas delivery systems, often utilize an actuating member made of a shape memory alloy (or SMA), e.g., nickel-titanium alloys. These alloys typically undergo a thermoelastic phase transition during passage from the martensitic to the austenitic phase, and vice versa, such transition being induced by a selected variation in temperature. Below the transition temperature (or martensitic phase), the alloy typically undergoes plastic deformation. The alloy remains in this condition until it is heated to a temperature above the transition temperature (or austenitic phase), at which time it reversibly recovers its original shape. In a typical valve configuration, a shape memory alloy member acts on a valve shutter against the bias of an elastic member, preferably made of steel. Such action on the shutter usually prevails when the temperature of the alloy member is lower than the transition temperature, whereas when its temperature becomes higher than the phase transition temperature, the shutter is overcome by the bias of the alloy member.
This configuration was used mainly to design on-off type valves: see for example U.S. Pat. Nos. 4,570,851, 5,261,597, 5,984,195, wherein the temperature of the shape memory alloy member is directly controlled by the flowing fluid which thus controls the opening and closure of the valve. In U.S. Pat. Nos. 4,736,587 and 4,973,024 the temperature of the shape memory alloy member is controlled by Joule effect by circulation of an electric current through it. In these examples the shape memory alloy member is in the shape of a wire or a coil spring.
Proportionally operating valves with an actuator made of a shape memory alloy heated by Joule effect have also been made. U.S. Pat. No. 5,865,419 discloses a valve shutter which is pulled towards the valve seat by a shape memory alloy wire against the bias of a steel spring. The passage of a controlled electric current through the shape memory alloy wire controls the force exerted on the shutter and therefore the flow section of the valve. The control unit includes a PWM controller to vary the duty cycle of the electric current to be supplied to the wire, thus controlling the amount of heat produced by Joule effect in the wire and therefore the variation of the valve opening following to a variation of the linear deformation of the wire.
U.S. Pat. No. 5,211,371 discloses a valve wherein the shutter actuator is constituted by a shape memory alloy wire arranged coaxially to a counteracting steel coil spring. The valve opening varies with the variation of the current circulating through the wire and provision is made for a control circuit based on the application of an electric drive to the SMA member to circulate a current through it capable to heat it and to operate the actuator. The drive is performed by means of a variable frequency pulse generator as a function of a control voltage applying pulses of the same duration with variable frequency. By increasing the frequency of the pulses the power applied to the SMA member, and then the amount of heating, is increased and vice versa. The control circuit can be associated to a closed loop feedback system.
A similar valve configuration with shape memory alloy actuator and relevant electric power control circuit is disclosed in GB 2251963. The control circuit uses a PID controller and is based, inter alia, on the variation of the resistivity of the SMA member as a function of the phase transitions of the constituent material.
While useful, valves with shape memory alloy actuators are limited in the attainable degree of controlled precision in response time that can be achieved. Response time must, of course, be as short as possible, especially when the SMA member is returned to the austenitic phase.